Failure diagnostic system for seat belt retractor

In order to provide a failure diagnostic system for a seat belt retractor which can reliably detect a failure of a motor of the seat belt retractor while the occupant feels no or little discomfort, a system for detecting a failure of a seat belt retractor having a motor (3) for rotating a webbing take-up spool (1) is presented, wherein the motor (3) is energized for a predetermined short period of time and, after stop of energization, induced current or voltage of wire-winding of the motor (3) is detected so as to determine a failure based on the induced current or voltage.